The present invention relates to improvements in methods and structures for providing highly wear-resistant, carbide-containing surfaces on tools or implements used for excavating, drilling, materials processing, materials handling, and other applications requiring substantial exposure to abrasion (referred to hereafter collectively as "tools"). More particularly, the invention relates to providing such a wear-resistant surface which is able to withstand a high degree of impact loading and thermal shock without cracking of the surface structure. The resistance to thermal shock permits hardening, by quenching, of a ferrous tool to which the wear-resistant surface structure has previously been attached by brazing or welding at a temperature tending to soften the tool, thereby providing the tool with strength and hardness in a manner compatible with the wear-resistant surface. Moreover, the surface structure has wear resistance superior to that of other carbide-containing surfaces, because wear-resistance is imparted to its matrix.
In the past, the construction of wear-resistant surfaces for tools of the type subjected to a high degree of abrasion has consisted of forming pads of carbide particles bonded together by a brazing matrix, and brazing such pad to the ferrous base metal of the tool, as shown, for example, in U.S. Pat. Nos. 2,833,638 and 3,882,594. All such methods and structures require that the ferrous base metal of the tool, where the pad is attached, be heated to a softening temperature in order to accommodate the brazing procedure, thus softening the base metal even though it may have originally been hardened by heat treatment with quenching. The result is a tool or implement whose wear resistance is improved, but whose strength and hardness are reduced, often causing deformation when the tool is subjected to impact loads. Such impact loads also cause the carbide particles to be torn out of the brazing matrix bonding them together.
In the course of developing the present invention, it has been discovered that the foregoing problems of prior art wear-resistant surface structures are due largely to their use of matrix materials for binding the carbide particles together which comprise copper alloys having substantial noncopper elements such as tin, zinc, cadmium, beryllium and the like. These noncopper elements, when used in combination with copper, form low melting point constituents within grain boundary locations in the matrix that lead to cracking upon a severe thermal shock treatment, such as quenching steel from an austenitizing temperature. Such cracking in turn leads to fracture when the surface structure is subjected to high-impact loads. Accordingly, to preserve the impact-resistance of the surface structure, quenching of the tool after attachment of the wear-resistant surface must be avoided at the expense of tool hardness.
Although the use of pure copper, as a brazing and matrix material, has long been known, its possible relevance to the aforementioned problem of incompatibility between the impact-resistance of carbide-containing surfaces and the hardness of their underlying tools has not previously been recognized.
Another problem of prior carbide-containing, wear-resistant surface structures involves their method of manufacture. Unless the surface structure is formed in a cavity or pocket machined into the tool or other base metal upon which the surface structure is to be mounted, it is exceedingly difficult to contain the carbide particles and matrix material on the base metal during the time that the assembly is in a furnace and the matrix material is in a molten condition. Carbide particles, particularly in large granular form, are intrinsically heavy and will roll off the edge of a part or slide down slight inclines unless restrained against doing so. Thus, without containment provided by machining a cavity into the base metal, it is difficult to obtain uniform distribution of the larger carbide granules over the surface of the base metal and, in particular, difficult to achieve uniformity in carbide particle distribution at the edges where the particles have a tendency to roll off. Unfortunately, if machined cavities in the base metal are employed to solve these problems, substantial extra cost incident to the machining is required Furthermore, after hardening of the carbide-containing surface structure, the outer edges of the cavity tend to protrude above the surface structure because of the volume collapse of the carbide and matrix mixture, requiring subsequent expensive grinding or machining to make the end product serviceable.
Other problems of prior wear-resistant surface technology include the limited methods and structures available for attaching the carbide-containing surface to a ferrous (steel or iron) tool, particularly with respect to effectiveness and convenience when the surface is attached to the tool after, rather than during, initial formation of the surface structure.
In addition, the prior art has paid little attention to the need for creating an environment for high flowability of the matrix during formation of the surface structure, so as to minimize voids and thereby maximize the integrity of the surface structure.